In regard to an enterprise or the like that has a business location in a plurality of districts, an extension communication network is built for each business location. Each extension communication network is built by providing private branch switching equipment and by connecting a telephone terminal located in the business location to the private branch switching equipment. In addition, when each private branch switching equipment is connected to each other, it is possible to perform a call between telephone terminals belonging to business locations different from each other. An extension number allocated to the telephone terminal has a number that indicates the business location, for example, a number for specifying the private branch switching equipment to which the telephone terminal is connected.
On the other hand, there is a service in which a mobile phone is made to be used as an extension telephone by connecting an existing mobile communication network to an existing extension communication network. According to this service, the mobile communication network is treated as an independent extension communication network.
In addition, a fact that identification information of a telephone apparatus that becomes a call origination and a telephone number of the call destination are stored in the telephone relaying apparatus in advance is disclosed in Patent Document 1. When receiving a telephone call from the telephone apparatus, the telephone relaying apparatus establishes this telephone connection, reads-out a telephone number corresponding to the identification information of the telephone terminal, makes a telephone call using this telephone number, and thereby performs the telephone relaying process.
For example, a method of controlling a third party call is disclosed in a Request For Comment (RFC) 3725. According to this method, when a mobile terminal makes an instruction on a call destination to the telephone relaying apparatus through a data communication network, the telephone relaying apparatus makes a call to the call destination and the mobile terminal that made a request.